In paper sheet handling, it is often necessary or desireable to make stacks or piles of sheets fed from some high speed duplicating or printing machine which have at least one stack registration edge surface as flat and sharply defined as possible. Furthermore, it is often desireable to separate the sheet output of a high speed duplicating or printing machine into off set job stacks, each job stack requiring its own well defined, and often highly defined, stack registration edge surfaces. A registration edge, in a horizontally piled stack of sheets, is a vertical surface of the side of the stack. Production of job stacks with highly defined registration edge surfaces is required where the job stack is to subsequently be bound without further stack preparation.
Some offset stackers, such as that disclosed in U.S. Pat. No. 4,817,934, make use of the force of gravity to urge successively piled sheets in a stack against a tote tray end stop to achieve stack registration end definition. Other offset stackers, such as disclosed in co-owned U.S. patent application Ser. No. 07/144,539, make use of a jogging device which jogs each successively received sheet of a stack of sheets against one or more vertically disposed stops to create a defined and substantially horizontal job stack. However, these offset stacking devices are not usually intended to produce the quality of defined stack edge that is sometimes required in certain paper handling applications, especially when asked to receive and create well defined stacks of sheets at high rates of sheet delivery to the stacker.
One of the difficulties with the above devices, and with other, conventional, stacking and offset stacking devices, results from a paper sheet's tendency to stick slightly to a sheet immediately below it in a stack. This is due, it is believed, partly to electronic charge accumulation in the sheets, and partly to frictive cohesion. Thus, jogging a top sheet will usually impart a moment of jogging momentum to one or more sheets immediately below the top sheet. This becomes particularly the case where a jogging mechanism applies necessarily greater force normal to the sheet in order to grip the sheet and pull it away from the sheet next below. This kind of "remedy" is thus somewhat self defeating. Another difficulty arises from the response of the sheet to its arrival upon the stack receiving surface and collision with the tote tray end stop. It tends to bounce back, to a degree largely dependent upon the sheet's beam strength and other factors as well, and to thereby form a misaligned stack registration edge as the sheets pile up.
Accordingly, there is a need for a device which can be used with a high speed offset stacking machine, and which accommodates the tendency of paper sheets to bounce back from their stops, and which furthermore positively aligns the trailing edge of each sheet with the stack registration edge surface after the sheet is received upon the stack. There is also a need for a device which reduces the tendency of a jogging mechanism to impart jogging momentum to sheets immediately below the sheet being jogged, and which provides a remedy to the tendency of paper sheets to stick slightly to one another during jogging, but without imparting jogging momentum to underlying sheets.